Correlation of physical and chemical properties of anhydride-cured epoxy resins with theoretical structure

The structure of alicyclic dicarboxylic anhydrides was found to have a significant effect on the final physical and chemical properties of a cured epoxidized novolac resin, and the final properties of a cured system can be predicted with reasonable accuracy from the theoretical structure. Properties were related to structure in terms of cyclic rings, unsaturation, substituted groups, etc. Measured properties were flexural strength, tangent modulus, toughness (area under the stress-strain curve), density, heat distortion temperature, and solvent sensitivity, as well as the retention of these properties after 250°C aging. The tensile test was substituted for the flexural test for the softer materials due to excessive slippage between supports.

The anhydride structure was found to influence properties primarily through its effect on physical density, cross-linking density, inter-conformational rotations, and extent of reaction. Overall results revealed hexahydrophthalic anhydride as an excellent hardener in the as-cured state for combining high toughness with good heat distortion temperatures and solvent resistance. Succinic anhydride is a close second in toughness although adversely affected in other properties by its low extent of reaction. Aging of the materials reveals the saturated single ring anhydrides to be superior in retention of properties, although all the materials embrittle severely, with methyl endomethylene tetrahydrophthalic anhydride having the lowest property retentions due to crazing.

Vinyl monomers were added to the unsaturated systems in an attempt to enhance properties by promotion of the reaction through unsaturation. These additions were found to produce no advantageous properties except viscosity reduction; an exception being the maleic anhydride cured system where the cross-linking rate was significantly increased.

The branched alkenyl succinic anhydrides were investigated as "flexibilizers" for a maleic anhydride cured system and the flexural strength, tangent modulus, toughness, and heat distortion temperatures were all found to decrease with increasing mole percent branched alkenyl succinic anhydride substituted for maleic anhydride. Properties of the substituted succinic anhydride cured materials, themselves, reveal that density, tensile strength, and moduli all decrease with increasing chain length while tensile elongation and solvent sensitivity increase. Essentially the effect of increasing chain length is to decrease both physical and cross-linking density and to subsequently affect all the other properties according to their dependence on these two density factors. Substituted succinic anhydrides with methyl branched alkenyl chains produced higher strength materials with higher heat distortion temperatures than the corresponding anhydrides with linear chains. The unsaturated substituted succinic anhydrides were found to be sensitive to surface embrittlement at high temperatures resulting in lower tensile elongations and toughness when compared to their saturated counterparts.